1. Field of the Invention
The present invention relates to an image transferring device for an image forming apparatus such as a copier, printer, facsimile transceiver or similar photographic image forming apparatus in which an image is formed on a photoconductive element. More particularly, the invention is concerned with a contact type image transferring device including, for example, a transfer roller or a transfer belt, for transferring a toner image from the photoconductive element to a sheet of paper which is passed through a nip between the photoconductive element and the image transferring device. The present invention further relates to a method and apparatus for electrically cleaning the transferring device which varies a force between the transfer device and the photoconductive element.
2. Description of the Related Art
It is a common practice for an image forming apparatus of the kind described above to use a contact type image transferring device. The contact type device transfers a toner image from a photoconductive element to a sheet passed through a nip between the photoconductive element and the transfer device to which an electrical field opposite in polarity to the toner image is applied. Since the contact type transfer device is in direct contact with the photoconductive element when the sheet is not at the nip portion, the toner image on the surface of the photoconductive element transfers to the surface of the transfer device. As a result, the toner image on the transfer device is transferred to the back side of the sheet.
Japanese Laid-Open Patent Publication No. 3-69978 discloses a cleaning device for a transfer roller in which toner on the surface of the roller is transferred to the photoconductive element by applying cleaning bias voltage to the transfer roller when the transfer roller is in direct contact with the photoconductive element. Since there is not only regular toner having a positive polarity but also oppositely charged toner having a negative polarity, for cleaning both types of toner, the related art discloses that the polarity of a cleaning bias voltage is switched over between the positive polarity and the negative polarity.
However, if a jammed sheet which carries un-fixed toner on it is in contact with the surface of the roller and a large quantity of toner is adhered to the transfer roller, it takes a long time to transfer all toner on the surface of the roller to the surface of the photoconductive element. As a result, users cannot use the image forming apparatus for a long period of time because during the cleaning process, the apparatus cannot operate.